Crystal Structures of Incommensurately Modulated Ln(PO<sub>3</sub>)<sub>3</sub> (Ln = Tb−Yb) and Commensurate Gd(PO<sub>3</sub>)<sub>3</sub> and Lu(PO<sub>3</sub>)<sub>3</sub>

2007-04-30T00:00:00Z (GMT) by Henning A. Höppe Stefan J. Sedlmaier
The crystal structure of the late lanthanoids' <i>catena</i>-polyphosphates Ln(PO<sub>3</sub>)<sub>3</sub> (Ln = Tb−Yb) is incommensurately modulated (Dy(PO<sub>3</sub>)<sub>3</sub>:  space group <i>Cc</i>(0β0)0; <i>Z</i> = 4; <i>a </i>= 1417.4(4), <i>b </i>= 670.96(14), <i>c </i>= 1009.5(3) pm; β = 127.62(2)°, <b>q </b>= 0.364<b>b*</b>; R<sub>all</sub> = 0.057, wR<sub>all</sub> = 0.071; 293 K) and consists of infinite chains of corner-sharing PO<sub>4</sub> tetrahedra. The cations are coordinated 6-fold in an almost octahedral arrangement over the whole modulation period. All atoms comprise a sinoidal positional modulation. The basic structure can be derived from a fcc packing which explains the pseudo-face-centering observed in the diffraction patterns. The crystal structure of Lu(PO<sub>3</sub>)<sub>3</sub> is isotypic with <i>C</i>-type phosphates (<i>Cc</i>; <i>Z</i> = 12; <i>a </i>= 1397.2(1), <i>b </i>= 2001.8(2), <i>c </i>= 995.56(9) pm; β = 127.351(6)°; R1 = 0.042, wR2 = 0.097, 293 K), and Gd(PO<sub>3</sub>)<sub>3</sub> crystallizes in a new structure type (<i>I</i>2/<i>a</i>; <i>Z</i> = 16; <i>a </i>= 2601.7(2), <i>b </i>= 1351.1(1), <i>c </i>= 1008.4(1) pm; β = 119.311(6)°; R1 = 0.039, wR2 = 0.092; 293 K). Both can be described in terms of superstructures of the basic structure unit cell of the incommensurate phases, and thus, a consistent structural description of many polyphosphates is provided. Tb(PO<sub>3</sub>)<sub>3</sub> was obtained as single phase adopting a novel synthesis under reducing conditions. The absence of an inversion center in the incommensurate phases and Lu(PO<sub>3</sub>)<sub>3</sub> was proved by a SHG experiment. The vibrational spectra are also discussed.